76 + Interview Questions in Dc Machines in Electrical Machines Page 2 InterviewSolution

51.	Compensating windings are embedded in pole faces and having the same polarity as that of adjoining armaturewinding.(a) True(b) FalseI had been asked this question in final exam.My question is based upon MMF and Flux Density Waveforms in DC Machines topic in chapter DC Machines of Electrical Machines
Answer» Correct OPTION is (b) False Easy explanation: Compensating WINDINGS are EMBEDDED in pole faces and having an opposite polarity as that of adjoining armature WINDING.

Discussion

52.	The brushes of a dc motor, are shifted by 5′, from the main field axis, then __________(a) demagnetizing or magnetizing effect may occur depending on machine(b) cross-magnetization will be pronounced(c) both cross-magnetization as well as magnetization and demagnetization will occur(d) none of the mentionedThe question was posed to me in exam.The above asked question is from MMF and Flux Density Waveforms in DC Machines in portion DC Machines of Electrical Machines
Answer» The correct ANSWER is (a) DEMAGNETIZING or MAGNETIZING effect may occur DEPENDING on machine For explanation I would say: When the brushes are shifted from GNA, there will either be magnetizing or demagnetizing effect will be observed.

Discussion

53.	The flux distortion caused by cross magnetizing armature reaction is more pronounced in a __________(a) dc shunt motor(b) dc series motor(c) any of the mentioned(d) differential Compound MotorsThis question was addressed to me in class test.Query is from MMF and Flux Density Waveforms in DC Machines topic in division DC Machines of Electrical Machines
Answer» Right choice is (a) dc SHUNT motor Explanation: Since field excitation remains SUBSTANTIALLY constant, while aarmature MMF reaches higher values at loading.

Discussion

54.	Which method needed to limit cross-magnetizing effect?(a) by increasing reluctance of cross flux path(b) by chamfering the pole faces(c) by compensating winding(d) any of the mentionedThe question was posed to me in a job interview.My doubt stems from MMF and Flux Density Waveforms in DC Machines in chapter DC Machines of Electrical Machines
Answer» The correct choice is (a) by INCREASING reluctance of CROSS flux path Explanation: By ultimately increasing the reluctance of the magnetic circuit at the pole tips so that the armature REACTION EFFECT is neutralized.

Discussion

55.	The air-gap flux density waveform has decreased flux under one pole tip and measured under the other is due to __________(a) cross magnetization(b) magnetization(c) demagnetization(d) saturation of iron magnetic circuitThe question was asked in an interview for job.My doubt is from MMF and Flux Density Waveforms in DC Machines in portion DC Machines of Electrical Machines
Answer» RIGHT choice is (a) CROSS magnetization For explanation: The FLUX density increases under one pole tip while it decreases under the other due to the saturation of IRON MAGNETIC circuit.

Discussion

56.	Armature reaction in a dc machine is __________(a) cross magnetizing in nature(b) magnetizing in nature(c) demagnetizing in nature(d) none of the mentionedThis question was posed to me in exam.This key question is from MMF and Flux Density Waveforms in DC Machines in division DC Machines of Electrical Machines
Answer» CORRECT answer is (a) CROSS magnetizing in nature Easy explanation: It is cross magnetizing in nature which happens due to interaction between stator and field MMF.

Discussion

57.	The armature mmf affects __________(a) commutation(b) generated voltage(c) torque(d) any of the mentionedI had been asked this question in examination.My doubt stems from MMF and Flux Density Waveforms in DC Machines topic in division DC Machines of Electrical Machines
Answer» RIGHT answer is (a) commutation To EXPLAIN: Armature REACTION THUS armature mmf affects all of the mentioned above.

Discussion

58.	A dc series generator as armature and field connections are reversed, the generator __________(a) stops(b) opposite direction(c) same direction(d) none of the mentionedThe question was asked by my school teacher while I was bunking the class.Question is from Methods of Excitation in division DC Machines of Electrical Machines
Answer» Right CHOICE is (a) stops To ELABORATE: It happens DUE to NON build up voltage.

Discussion

59.	_____________ is responsible for mechanical power output of a DC motor.(a) Electrical input power(b) Any of the mentioned(c) Air-gap flux(d) Armature emfThis question was posed to me in a national level competition.My question is from Methods of Excitation topic in section DC Machines of Electrical Machines
Answer» Right answer is (a) ELECTRICAL INPUT power Best explanation: PRIMARILY it is the electrical input power that starts the motoring ACTION.

Discussion

60.	The number of parallel paths in the armature is increased by __________(a) increasing number of magnetic poles(b) decreasing number of magnetic poles(c) lap number of magnetic poles(d) using more brushesI got this question by my college director while I was bunking the class.Question is from Methods of Excitation topic in chapter DC Machines of Electrical Machines
Answer» The correct choice is (a) INCREASING number of magnetic poles For EXPLANATION I would say: It is increased by increasing magnetic poles which give parallel PATH to the magnetic circuit.

Discussion

61.	A series generator, having external characteristics which is a straight line through 0 to 50 V at 200 A is connected as a booster, between a station bus-bar and feeders of 0.5 ohm resistors. The voltage at the far end of feeder at current of 50 A is?(a) 25 V(b) 17 V(c) 8 V(d) 50 VThe question was asked in a national level competition.The doubt is from Methods of Excitation in portion DC Machines of Electrical Machines
Answer» RIGHT answer is (a) 25 V Explanation: V = VOLTAGE drop-voltage added up by BOOSTERS = (0.350 – 5050/200) = 15-12.50 = 2.5 V.

Discussion

62.	Flashing is used to __________(a) temporarily excited field from a battery source for large DC generator(b) temporarily excited field from a battery source for small DC generator(c) temporarily excited field from a battery source for large DC motor(d) temporarily excited field from a battery source for large DC motorI had been asked this question in semester exam.This question is from Methods of Excitation topic in portion DC Machines of Electrical Machines
Answer» Correct choice is (a) TEMPORARILY EXCITED field from a battery source for large DC generator Easy explanation: It is a method to give a kick start to dc machine when its residual magnetism is VANISHED.

Discussion

63.	If a DC shunt generator fails to start, the most primitive action would be __________(a) reversing field connection to armature(b) reversing direction of rotation(c) any of the mentioned applicable ways(d) none of the mentionedThis question was posed to me in exam.The question is from Methods of Excitation in portion DC Machines of Electrical Machines
Answer» The correct choice is (a) reversing field connection to armature Easiest EXPLANATION: The first measure is to CHECK if the field winding as properly connected with ADDITIVE polarity so that the flux is aiding in NATURE.

Discussion

64.	The critical resistance refers to __________(a) the resistance above which machine does not excite(b) the resistance below which machine does not excite(c) the resistance at which machine does not excite(d) any of the mentionedI had been asked this question in my homework.Enquiry is from Methods of Excitation topic in portion DC Machines of Electrical Machines
Answer» Right answer is (a) the RESISTANCE above which MACHINE does not excite The explanation is: After taking a glance at the magnetization curve, it can be concluded that the emf will not build up if the resistance is GREATER than critical resistance.

Discussion

65.	The self-excited dc generator with gradual build up of residual voltage and EMF corresponds to __________(a) positive feedback(b) negative feedback(c) saturation(d) any of the mentionedI had been asked this question in an interview.My doubt is from Methods of Excitation in chapter DC Machines of Electrical Machines
Answer» The correct choice is (a) positive FEEDBACK To elaborate: It RESPONDS to a positive feedback because the gain is always more than ONE in the linear region.

Discussion

66.	A shunt generator has an induced voltage on open circuit of 127 V. When the machine is on load the terminal voltage is 120 V. The load current if the field resistance be 15 ohm and armature resistance be 15 ohm.(a) 342 A(b) 350 A(c) 358 A(d) 8 AI got this question in an interview for internship.Origin of the question is Circuit Model of DC Machines topic in chapter DC Machines of Electrical Machines
Answer» Right answer is (a) 342 A To explain I would say: Ia*Ra = E-V = 127 – 120 = 7 V Ia = 350 A Ish = 120/15 = 8 A I = 350 – 8 = 342 A

Discussion

67.	If a self excited DC generator is failed to run, this refers to ________________(a) zero residual voltage(b) field MMF, that it is not cumulative(c) resistance is greater than critical resistance(d) any of the mentionedThis question was posed to me in examination.This interesting question is from Methods of Excitation topic in portion DC Machines of Electrical Machines
Answer» The CORRECT option is (a) zero residual voltage To elaborate: Any of the REASONS are VALID for no BUILD up of an emf.

Discussion

68.	Consider a DC generator running at the rated speed of 2000 rpm, suddenly there is an insulator falls on the field circuit and breaks it. Then __________(a) the motor stops in a few rounds(b) it continues to run but as dc motor(c) it continues to run as a motor(d) none of the mentionedThis question was posed to me in an online quiz.My doubt stems from Methods of Excitation in division DC Machines of Electrical Machines
Answer» Correct answer is (a) the motor stops in a few rounds Easiest EXPLANATION: As the field circuit GETS broken there is interruption in the flux to the dc machine, and as a RESULT the motor stops in few SECONDS.

Discussion

69.	Consider a 200V, 25kW, 30A DC machine lap connected with armature resistance of 0.4 ohms. If the machine is later wave wound, then the developed power is?(a) 25 kW(b) 12.5 kW(c) 20 kW(d) 50 kWI had been asked this question in homework.Enquiry is from Circuit Model of DC Machines topic in portion DC Machines of Electrical Machines
Answer» Correct CHOICE is (a) 25 kW Best EXPLANATION: The power of the MACHINE remains unaltered by the type of CONNECTIONS.

Discussion

70.	If the DC machine is held constant at 3000 rpm. The DC voltage is 250V. If the field is held constant with 250V. Is this machine generator or motor?(a) Motor(b) Generator(c) None of the mentioned(d) Any of the mentionedThe question was asked in my homework.My question is based upon Circuit Model of DC Machines in division DC Machines of Electrical Machines
Answer» The CORRECT ANSWER is (a) Motor Best explanation: From the speed and EMF relation, E = 250*2950/3000 = 245.8 V This is less than the terminal voltage. HENCE it is a motor.

Discussion

71.	If the armature terminal voltage is more that its induced EMF, the DC machine given is ________(a) motoring mode(b) generating mode(c) regenerative mode(d) none of the mentionedI got this question by my college professor while I was bunking the class.This key question is from Circuit Model of DC Machines topic in chapter DC Machines of Electrical Machines
Answer» Right OPTION is (a) MOTORING mode Easy explanation: As the TERMINAL voltage is LESSER than armature voltage, the supply is fed to the machine and so it will be acting LIKE a motor.

Discussion

72.	The conductor EMF and current are in _____ direction and developed torque is in _____ for generating mode.(a) same, opposite(b) same, same(c) opposite, same(d) opposite, oppositeThe question was asked by my school principal while I was bunking the class.I'd like to ask this question from Circuit Model of DC Machines in section DC Machines of Electrical Machines
Answer»

Discussion

73.	If the electromagnetic torque in a DC shunt-generator is opposite, what can be further concluded?(a) Mechanical power is absorbed by the machine(b) Mechanical power is delivered by the machine(c) Electromagnetic torque is in same direction of prime mover(d) None of the mentionedThe question was posed to me in an online quiz.Origin of the question is Circuit Model of DC Machines in chapter DC Machines of Electrical Machines
Answer» The CORRECT option is (a) Mechanical power is ABSORBED by the machine The BEST explanation: When the electromagnetic torque is in opposite DIRECTION, it is of motoring nature.

Discussion

74.	If the terminal voltage of 220-V dc generator having armature resistance of 1 ohms. The induced emf produced is 200-V. The armature current for the above machine is?(a) 20 A(b) -20 A(c) -10 A(d) 10 AI had been asked this question during an online exam.Query is from Circuit Model of DC Machines topic in portion DC Machines of Electrical Machines
Answer» The CORRECT OPTION is (a) 20 A To ELABORATE: I = V-E/r = 220-200/1 = 20 A.

Discussion

75.	What is the shaft power at the DC Generator?(a) sum of mechanical power and rotational losses(b) only mechanical losses(c) no-load losses and mechanical power(d) any of the mentionedThis question was addressed to me during an interview.My query is from Circuit Model of DC Machines topic in chapter DC Machines of Electrical Machines
Answer» Correct OPTION is (a) sum of MECHANICAL POWER and ROTATIONAL LOSSES Easy explanation: The shaft power is sum of mechanical power and rotational losses.

Discussion

76.	The voltage drop at brush-commutator contact is variable (1-2V) and dependent of armature current.(a) True(b) FalseI have been asked this question in an interview for job.I'd like to ask this question from Circuit Model of DC Machines in chapter DC Machines of Electrical Machines
Answer» RIGHT answer is (B) False For explanation: The VOLTAGE drop at brush-commutator CONTACT is variable (1-2V) and INDEPENDENT of armature current.

Discussion

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The armature mmf affects __________(a) commutation(b) generated voltage(c) torque(d) any of the mentionedI had been asked this question in examination.My doubt stems from MMF and Flux Density Waveforms in DC Machines topic in division DC Machines of Electrical Machines

The voltage drop at brush-commutator contact is variable (1-2V) and dependent of armature current.(a) True(b) FalseI have been asked this question in an interview for job.I'd like to ask this question from Circuit Model of DC Machines in chapter DC Machines of Electrical Machines